Modern radar and communications systems often employ waveforms significantly more complex than the simple modulation schemes of the past. This increased waveform complexity results from the need to operate in crowded and sometimes hostile signal environments. In radar applications, a need exists to separate and identify target echo signals deeply embedded in clutter. Radar systems are often provided with variable modulation formats for use in different tactical situations. These developments have lead to the need for programmable signal processors which can function as matched filters or cross-correlators for maximizing signal-to-noise ratios.
A chirp transform correlator can be used to perform both auto-correlation and cross-correlation. An example of the latter function would be the correlation of a noisy radar target echo signal with a reference signal comprising a sample of the radar's transmitted signal. Such correlators make use of the fact that correlation of two signals is the equivalent of taking the inverse Fourier transform of the product of the Fourier transform of one of the signals and the complex conjugate of the Fourier transform of the other signal. Fourier transformation can be conveniently implemented with the use of sweeping local oscillators in conjunction with chirp filters comprising state of the art devices such as reflective array compressor (RAC) filters.
Prior art chirp correlators have utilized a single signal processing channel to obtain the Fourier transform of the unknown signal. Such a correlator was built and tested to prove the concept of this correlator architecture, however such circuitry required that the timing and duration of the unknown input signal be coordinated with the timing of the sweeping local oscillator with which the input signal was mixed before application to the chirp filter. In most tactical situations, such a correlator must be asynchronous in that it must be capable of processing signals of arbitrary or random time of arrival. The present invention provides such asynchronous correlation.